Chemicals used to keep swimming pools clean may be behind the rise in asthma in children, a study suggests.

Researchers in Belgium have found that chlorine in pools can react with sweat or urine to create harmful fumes which can damage lungs.

They believe exposure to these fumes leaves children susceptible to allergens, which may then trigger asthma.

But other experts have criticised the study and have dismissed the claims.

Dr Alfred Barnard and colleagues at the Catholic University of Louvain in Brussels carried out tests on 226 primary school children who had swum regularly - either weekly or fortnightly - in indoor pools from a young age.

They also carried out tests on 16 children, between the ages of five and 14, and 13 adults, aged between 26 and 47.

It is unlikely that swimming, at least by itself, could really be the cause for the increase in asthma

Professor Martyn Partridge, National Asthma Campaign

They measured levels of key lung proteins in all of these people. The proteins they measured are known to cause damage to cells in lungs if they are present in high quantities.

The researchers found that children who regularly attend indoor pools accumulate these proteins, making them more at risk from asthma.

'Minimum exposure'

They found that children who swam most frequently had protein levels similar to people who smoke regularly.

Protein levels were also high in people who had been sitting at the poolside and had not swum.

While such high levels do not in themselves cause asthma, they may increase the risks of lung damage.

The researchers suggested that they could increase the risks of children developing asthma.

But they acknowledged that further research was needed to confirm their theory.

Nevertheless, they suggested that there may be a case for pools to be cleaned with non-chlorinated disinfectants.

Writing in the journal Occupational and Environmental Medicine, they said: "The question needs to be raised as to whether
it would not be prudent in the future to move towards non-chlorine based
disinfectants or at least to reinforce water and air quality control in indoor
pools in order to minimise exposure to these reactive chemicals."

'Distorted'

But Professor Martyn Partridge, of the UK's National Asthma Campaign, said the findings needed to be interpreted carefully.

"It is unlikely that swimming, at least by itself, could really be the cause for the increase in asthma.

"Taking part in swimming may just be a marker of a type of lifestyle associated with an increase in asthma," he said.

The results may have been distorted by families with a history of asthma or other family members with asthma being more likely than others to go swimming.

This was because of "the widespread knowledge that exercising by swimming is less likely to provoke attacks of asthma in those with the condition than other types of exercise", he said.

An estimated one in eight British children suffers from asthma, while one in five has been diagnosed with the condition at
some stage in their lives.